[Note: ug stands for microgram, or one thousanth of a miligram. Most supplements express dosages in international units (iu) – which is an international standard of an agreed upon biological effect. This varies by the substance measured. For vitamin D3, the conversion to iu is ug times 40; so 22 ug/day – the amount given to these young women starting in September - translates to 880 iu/day. Blood testing for 25(OH)D is considered the best indicator of vitamin D status.]

The serum 25-hydroxyvitamin D [25(OH)D] response to daily supplementation with 20 ug cholecalciferol (D3) during winter in predominantly white premenopausal women living in Maine was measured and the effects of body composition and hormonal contraceptive use on baseline serum 25(OH)D concentrations and the response to supplementation were examined.

A total of 112 women (22.2 ± 3.7 years old) received placebo from March 2005 until September 2005 when they were randomized to receive either placebo or 20 ug/d D3 through February 2006. Eighty-six women completed the study.

Serum 25(OH)D concentrations increased by 35.3 ± 23.2 nmol/L from February 2005 to February 2006 in the treatment group, significantly more than the 10.9 ± 16.9 nmol/L increase in the placebo group. Treatment group, magnitude of summer increase in 25(OH)D, estrogen dose, and baseline serum 25(OH)D concentrations, but not body fat, were significant predictors of the 1-year change in 25(OH)D concentrations used to assess the magnitude of the response to supplementation.

Daily supplementation with 20 ug D3 during winter achieved optimal 25(OH)D concentrations (75 nmol/L or more) in 80% of participants, indicating that this dose is adequate to optimize vitamin D status in most young women in Maine.